The purpose of this project will be to develop a system for measuring oxygen concentrations on and inside the eye. This instrument will take advantage of phosphorescent probes that are sensitive to oxygen concentrations. This will have obvious applications in eye research and may eventually have applications in disease diagnosis. Our technique uses a dual frequency photon counting system, which we have developed specifically for this purpose. It combines maximum sensitivity with the ability to measure a wide range of oxygen concentrations. During Phase I, we will demonstrate feasibility of this technique both in vitro and in the rabbit eye. We will perform most of our work with a well established dye system, but will also begin to characterize additional dyes for further development during Phase II Our overall goal will be to develop multiple measurement systems based on dyes that have been selected for specific environmental affinity, oxygen range, etc. A critical goal will be standardization and calibration control, so that results made on different machines will be comparable. In addition, we will design this instrument with enough flexibility so that we can add capabilities as new phosphorescent oxygen probes are developed.